Various fluid filtering devices have been proposed over the years, the more common types employing a porous filter media which is penetrable to the fluid to be filtered but substantially impenetrable to contaminants to be filtered out. A flow of fluid is directed through the media such that contaminant particles which do not pass through the pores of the filter media are retained on or in the media on the upstream side. Purified fluid passes through the media and on to its end use downstream. The pore size of the media determines the fineness of filtration or, conversely, the size of the contaminant particles which pass through the media and are not filtered out. After a period of filtration, contaminants collected on the upstream side of the media plug or clog the media such that fluid flow through the media is reduced and/or the pressure differential between the fluid on the upstream side of the media and the fluid on the downstream side is increased to an unacceptable level. Decreased flow translates to a reduction in purified fluid output, whereas an increase in differential pressure generally signifies greater strain on the fluid systems producing the fluid flow to the filter, such as pumps, piping and seals, accelerating their wear and consuming more energy. For this reason, fluid filtration systems typically have some provision for unclogging the filter media at regular intervals. This may take the form, inter alia, of reverse flushing the media to waste, replacing the media with new media, subjecting the media to some form of mechanical cleaning process such as scraping or brushing, or a combination of the foregoing methods, depending upon the media employed.
There are numerous kinds of filter media, with each type having its advantages and disadvantages with respect to filtering efficiency, backwashing/cleaning effectiveness and cost. For example, cartridge filters typically employ a filter element fabricated from a fibrous or woven sheeting material, such as a paper, felt, fiberglass, woven fabric or screen-like material surrounding a central, perforated core cylinder and capped with end plates. Cartridge filters are light in weight, compact and effective at removing small particulates. Fluid flow is commonly directed from the outside of the cartridge element, which usually approximates a cylindrical shape, to an inside core cylinder. The fluid permeable sheeting typically serves as the filter media itself and may be backflushable. In order to increase the filter media surface area, it is known to fold the media sheet in a continuous zig-zag or accordion pattern. Cartridge filters have certain inherent drawbacks, such as becoming clogged by fine particulates or organics that resist backflushing. Cartridges also tend to collapse as the flexible media sheet experiences greater pressure differentials. This collapse may take the form of a general deformation of the overall cylindrical shape of the cartridge or the collapse of the peripheral folded zig-zag pattern such that the surface area advantage provided by the folding is defeated. Cartridge collapse has been remedied in the past by the inclusion of a parallel metal screening bent in the same shape as the media sheeting. Solutions for the latter problem of the pinching down of the folds have been proposed in the form of corrugated fold separators made from rigid plastics as shown, e.g., in U.S. Pat. Nos. 4,075,106 to Yamazaki and 4,560,477 to Moldow. Because cartridge media becomes blocked under certain circumstances, the only remedy to restore the media is replacement, which is both expensive and inconvenient, in that the cartridges are the products of rather complex fabrication methods and to replace them, the filter must be disassembled.
As an alternative to filtering media in the form of woven or flocked sheeting, many filtering systems use a granular or particulate media, such as sand or diatomaceous earth, hereinafter "DE", as the primary media for collecting contaminants. The primary media is prevented from entering the fluid flow by an element which is porous to the fluid to be filtered yet impenetrable to the granular media, e.g., a fine mesh screen or a sintered filter block. The granular media collects the contaminants from the fluid stream thereby protecting the media-impenetrable element from becoming clogged by contaminants. That is, the fluid to be filtered passes through a filter bed or cake of granular media and then through the media-impenetrable element, on to its final use downstream. For example, a DE filter typically contains at least one septum which is a porous, fabric-like element that is penetrable to the fluid to be filtered but not to DE. The septum has pores with dimensions which preclude the DE from entering the filter stream in significant amounts after a precoating of DE has been deposited thereon. Usually, the pore size of the septum is larger than the particle size of the DE to promote flow and to insure that the finest filtration takes place in the DE precoat. Even though the primary media has particles which are smaller than the septum pores, unacceptable leakage of primary media through the septum is avoided due to the interlocking and agglomeration of DE particles to form groups which are larger than the pores in the septum. In this manner, the DE precoating or filter cake protects the fine pores of the septum from blockage by dirt. It is therefore desirable, for any given grade of DE, that the septum holes be significantly larger than the DE particle size. Septum pores that are too large however, result in unacceptable leakage of DE past the septum and prolonged periods of time for establishing a precoat. DE is deposited upon the septum by introducing it into the fluid stream, typically in a slurry, such that it is deposited upon the septum by fluid flow therethrough forming a filter cake or layer of DEL on the septum. The septums of DE filters may take various forms and are frequently disposed about a spacer of some sort which internally supports the septum as e.g., shown in U.S. Pat. Nos. 3,774,772 to Yeths (arcuately-shaped), and 3,100,190 to Hobson, Jr. (tube-shaped).
The use of granular filter media like DE and sand has certain benefits, e.g., both are relatively inexpensive materials and tend to pack quite closely giving fine filtration results. As with all filters, however, after a period of use, contaminants and dirt accumulate on the upstream side of the granular filter media. Granular media typically has a range of particle sizes and shapes, thus giving rise to a spectrum of pore sizes in any given sample. Smaller dirt particles have a greater tendency to penetrate the surface of the granular media bed or cake until being trapped by a smaller pore below the surface. This gives rise to a contaminated band of media extending from the surface into the depth of the media. Eventually, the accumulation of dirt causes a resistance to flow and an increase in operating pressure indicating a need to change the filter media or to clean the accumulated dirt therefrom in order to permit a resumption of normal flow rates. A common method for cleaning dirt from DE filters is by backwashing, wherein a reverse flow of fluid is directed through the septum to dislodge accumulated dirt, as well as, the DE filter cake from the septum. The reverse backflushing flow, with dirt and DE filter media included, is directed to waste, such that there is a loss of fluid and DE associated with a removal of dirt from the filter. In this day of ever increasing eco-consciousness, the disposal of DE in the course of backflushing filters, e.g., on swimming pools, has received increased negative scrutiny.
As an alternative to backflushing, it has been recognized that the DE filter media may be given extended filtering life by reorienting the filter cake on the septum without backflushing or disposing of the DE. For example, U.S. Pat. No. 5,013,461 to Drori discloses a DE filter with a centrally located piston for creating reverse flows for dislodging DE filter cake from the exterior surface of a laminated disk filter element. Similar piston arrangements are shown in U.S. Pat. Nos. 4,156,651 to Mehoudar and 1,994,656 to Liddell. U.S. Pat. No. 3,735,872 to Anderson discloses apparatus for regenerating the DE precoat using the resiliency of foam septum elements underlying the precoat, by scraping and by classifying the dislodged DE into a precoat of graduated particles. Even without the affirmative step of classifying, reorienting, "opens up" the filter cake by breaking up the surface coating of dirt that clogs the DE filter cake. Reorienting disperses the dirt throughout the DE layer taking advantage of the improbability that the filter cake and dirt will reassume an orientation of particles packed so closely together as to unduly restrict fluid flow therethrough. The step of reorienting the DE without backflushing involves a cessation of filter operation, disturbing the DE filter cake and filtered dirt from the septum in some manner, and resuming filtering operation, such that the DE and the dirt are redeposited upon the septum in a new orientation. Reorientation will typically result in some portion of the DE and some portion of fine dirt which is smaller than the pores in the septum being introduced into the filtered fluid stream unless the fluid flow is directed to waste upon restarting.
Another limitation associated with DE filters is the tendency for the DE to coat the septum unevenly, wherein some portions are not coated sufficiently and other portions are coated excessively. This is due to the "bridging" of DE between adjacent surfaces of the filter element or "drifting" of the DE, creating areas of the septum where a thick impervious layer of DE has been deposited due to the septum shape and its interaction with the hydrodynamic flows within the filter.
It is therefore an object of the present invention to provide an improved fluid filter having an element which exhibits an increased filtering surface area that is resistant to collapse or reduction of filter surface area under high differential pressures. It is a further object to provide a diatomaceous earth filter which avoids bridging and is provided with apparatus to regenerate the DE precoat without backwashing.